Recently in the semiconductor industry, there are rapid developments towards higher integration and higher precision in semiconductor memory devices, and as a result, the importance of technologies related to air cleanliness and micro-vibration control is also being emphasized.
An exposure apparatus, which is an essential part of high-tech semiconductor processes in particular, is an apparatus for drawing circuit patterns by irradiating rays onto a circular semiconductor plate known as a wafer. The semiconductor exposure process entails passing the rays through a mask in which the circuit patterns are provided, so that the rays draw the circuit patterns onto the surface of the wafer on which a film of photosensitive liquid is applied. As the rays are irradiated after the mask is placed over the wafer, the rays that pass through the circuit patterns transcribe the circuit patterns onto the wafer.
Here, the exposure process is performed by way of an exposure apparatus called a stepper. When the mask is placed on the stepper and the rays are passed, fine-scale drawings of the electronic circuits are formed over the wafer coated with the photosensitive liquid.
In order to form circuit patterns in an even finer scale, the resolution, also known as the resolving power, of the exposure apparatus has to be increased. The resolving power or resolution refers to the ability of recognizing two adjacent objects as being separate from each other. It is mainly used in representing the performance of an optical device, and in the context of an exposure apparatus for carving circuit patterns onto a wafer, this ability evaluates ‘how fine a scale the circuit patterns can be formed in’. Attempting to draw a fine pattern that exceeds the limit of the resolution would incur interference due to the diffraction and the resulting scattering of the rays, so that a distorted image different from the original mask pattern would be formed on the wafer. A chip having a distorted pattern would not be able to function properly.
Until now, a semiconductor exposure apparatus would increase the resolution by using a lens having a higher numerical aperture (NA) or by using a light source of a shorter frequency. Up to the 130 nm semiconductor, a krypton fluoride (KrF) excimer laser having a wavelength of 248 nanometers (nm) was used, and from the 90 nm scale onwards, an argon fluoride (ArF) excimer laser having a wavelength of 193 nm was utilized. With these procedures, various resolution-increasing technologies for reducing interference from the diffraction and scattering of rays were introduced. Some examples of such technologies include the phase shift mask (PSM), which adjusts the intensity and phase of the rays to eliminate diffracted light, and the optical proximity correction (OPC) mask, which artificially modulates patterns where distortions are anticipated so that the correct image is obtained.
The ArF exposure apparatus currently employed in the latest semiconductor production lines utilizes immersion technology, which increases resolution by using a liquid medium having a refractive index (1.44) higher than that of air. However, if the gate line width decreases to 30 nm or lower, the physical ability of the immersion ArF exposure apparatus to implement circuit patterns also reaches its limit.
Gaining attention as a new alternative are extreme ultraviolet (EUV) rays, which is an electromagnetic wave in an intermediate range between ultraviolet (UV) rays and X-rays. The EUV ray intended for use in semiconductor processes was designed to have a wavelength of 13.5 nm. By utilizing the EUV ray having a short wavelength, it is possible to produce semiconductors of lower than 10 nm with a single exposure rather than by multi-patterning. Currently, the only manufacturer developing EUV equipment is the Dutch company ASML, which holds the number one ranking in the semiconductor exposure market.
Korean companies have also started adopting exposure apparatuses that utilize such EUV rays, and when using the EUV exposure apparatus, the patterning operation needs to be performed just once in contrast to the multi-patterning operation required for existing apparatuses using ArF, so that the patterning cost is greatly reduced. When using a 15 mJ dose photosensitive liquid (where millijoule dose is a unit representing how much irradiation is needed for photosensitive response, with a higher value requiring greater irradiation), the cost of the patterning by EUV rays is less than 300% (with the cost of immersion ArF regarded as 100%).
Whereas, in the past, the lattice beams for semiconductor factories were designed to accommodate micro-vibration control in the order of sub-micrometers due to the fact that the wavelength of argon fluoride (ArF) in existing ArF exposure apparatuses is 193 nm, the introduction of the latest exposure apparatuses utilizing EUV rays means that logic chips of less than 10 nanometers can be produced, whereby there is now a demand for vibration control at the level of several nanometers.
Technologies related to micro-vibration control in semiconductor factories can be classified mainly into methods regarding the design of low-vibration building structures, the arrangement and separation of semiconductor manufacturing equipment, vibration-proofing of the vibration source, vibration control of machinery sensitive to vibration, and others. Also, an overall evaluation of the impact of vibrations must be performed from the initial design stage to the final stage of producing a prototype.
A unique structure closely associated with vibration control in a semiconductor factory is the lattice beam, which is used for air cleansing. On this lattice beam structure, an independent vibration control table is installed, and on the independent vibration control table, precision semiconductor production facilities are placed.
Generally, regulations on the permitted levels of vibration in semiconductor factories are also provided in 3 dimensions for different frequencies for both the access floor and the vibration control table, which is the independent isolated foundation on which high-precision exposure apparatuses such as the stepper, aligner, etc. are installed.
When designing a conventional lattice beam, the damping ratio of the structure is fixed to a predetermined value appropriate for the properties of the structure, while the stiffness and mass are set as variables. In other words, in order to effectively control the dynamic response of the lattice beam, the stiffness of the lattice beam structure has to be increased so that the vibration displacement is decreased, and the natural frequency of the lattice beam must be designed higher than the frequency of the external vibration source. Since a high stiffness is needed for satisfying these requirements, a large size is unavoidable.
In such cases, in order to accommodate the level of vibration control performance in the order of several nanometers as needed for introducing the latest exposure apparatuses as described above, the depth of the lattice beam has to be increased. That is, the depth has to be increased by at least twice the depth of the current lattice beam.
However, increasing the depth of the lattice beams would affect the height of the building and would lower manufacturability due to the problems of increased facility sizes, increased curing times, increased difficulty in removing the concrete molds, increased sizes of the lifting and transporting equipment, etc. Also, limitations in the lifting weight would require that structures be divided into smaller modules, leading to increased numbers of lifting steps and decreased workability.
In particular, while the Republic of Korea is expected to become the largest adopter of the latest EUV exposure apparatuses, problems in construction are avoidable at semiconductor facility factories unless the technical problem of enabling ultrafine vibration control is resolved.
Thus, even though the introduction of the latest exposure apparatuses would enable the production of ultra-precision high-integration semiconductors in the order of several nanometers and also allow a twofold increase in productivity compared to existing apparatuses to permit a position of considerable competitive advantage in the global semiconductor market, the current lack of research on lattice beam technology that can realize vibration suppression at the nanometer level poses a technical obstacle. As such, there is an urgent need for lattice beam technology that can overcome this obstacle and realize ultrafine micro-vibration suppression.
[Related Art Document] Korean Patent No. 10-0392188 (Registered Jul. 8, 2003)